Hi and welcome.
Different size needles are used for different size yarns, some are thick and some are thin. Get a medium one, like Jan saidn and though the label may say a size 8 needle is used with it, I suggest a little larger one, like a 9 or 10 because it makes bigger stitches and they're easier to see. It should be light or multicolored - it's harder to see the stitches on dark ones.
The needles attached to a cord are called circulars and you can use them to learn on. I think they're easier because you're not fighting to balance the wieght of the straight ones at the same time you're learlning to hold the needles and yarn and wrap the yarn to form the stiches. You can use them just like straight ones and also for knitting things in the round without seams. A 29-32" length is good, they're long enough for adult sweaters and small blankets, but not too long for knitting small things like scarves and dishcloths.
sue- knitting heretic